1. Field of the Invention
The present invention relates to failure analysis and more particularly to a method and apparatus for analyzing defects or verifying structures in electrical components such as microcircuits.
2. Background Information
Conventional examination of structures, such as electrical components, is often accomplished by the use of x-ray systems. An example of an x-ray system that has been used for this purpose is the Fein Focus FSX-100 system, which is a transmitted real time x-ray system.
In a Fein Focus FSX-100 system a sample structure to be examined is positioned near the x-ray tube. The x-ray tube generates an electron beam, which is accelerated to a high velocity while traversing an electric field, and is directed towards a beryllium wafer located at the end of the tube. The beryllium wafer has a tungsten plating coated on it. Because this tungsten plating constitutes the target for the electron beam, it represents the target material for the accelerated electron beam. Upon striking the tungsten plating, x-rays are generated. The generated x-rays travel through the remaining tungsten plating, made up of tungsten atoms, and through the beryllium wafer leaving the vacuum environment of the x-ray tube.
The depth of penetration of the electron beam into the target material varies with the acceleration voltage. X-ray machines are typically designed so the acceleration voltage can be varied. A problem with this arrangement is that the target material must be thick enough to accommodate all acceleration voltages. By design, x-ray systems therefore use a target material that is thicker than needed. The electron beam thus penetrates a small percent of the target thickness when lower acceleration voltages are used. The x-rays that are generated must then pass through the remaining target thickness, which results in loss of contrast as the size of the sample structure is decreased.
The use of this x-ray system can thus lead to an invisible contrast pattern. For example, where the electron beam is approximately 3 to 5 microns in diameter when it hits the target, the system""s resolution is inadequate for examining microcircuits where the spacing between the circuitry is less than the 3 to 5 micron diameter of the beam.
Contrast is further diminished when the x-rays travel through other materials. That is, once x-rays have penetrated the sample structure, they enter an image intensifier tube, which is another vacuum device like the x-ray tube. The x-rays travel through the outer support material of the image intensifier tube, which can, for example be formed as an aluminum baseplate which is about 10 inches in diameter. The x-rays enter the image intensifier tube where they are converted into light, and then into electrons utilizing both phosphor and photo cathode materials. The electrons are then amplified and converted by a phosphor into light photons where a charged coupled device (CCD) video camera displays the image on a monitor.
Because the x-rays travel through materials other than the sample structure, the system""s contrast is limited. In addition, the electron beam diameter limits the system""s resolution. Present x-ray systems can magnify up to 300 times but this again only provides limited enhancement. Because the sample structure is located in the x-ray chamber where no vacuum is present, this also limits the ability to improve resolution. The foregoing limitations preclude accurate examination of small structures, such as the circuitry inside microcircuits.
Thus, it would be desirable to provide systems and techniques for accurately examining microcircuits.
The present invention is directed to an apparatus and method for examining small structures such as microcircuit structures. Exemplary embodiments can generate narrow electron beams for examining the microcircuits, and can eliminate the traversing of the x-rays through materials other than a target material and the sample structure. Exemplary embodiments eliminate having the x-rays traverse both a wafer and a plating target on the wafer by using a target material of proper thickness placed closer to the sample structure. Exemplary embodiments provide adequate magnification to perform fault analysis of small structures and provide an adequate contrast pattern for viewing images of the microcircuit being examined.
Generally speaking, the present invention relates to a method and apparatus for inspecting a structure. Exemplary embodiments comprise means for supporting a target material; means for supplying an electron beam to the target material to convert the electron beam into x-ray radiation, which is emitted directly into a structure to be inspected; and means for producing an image using x-ray radiation emitted from the structure to be inspected.
In other exemplary embodiments, an apparatus for inspecting a structure comprises an electron beam generator for supplying an electron beam to an inspection area; a target material for generating x-rays in response to the electron beam; an x-ray phosphor, located a predetermined distance from at least a portion of the inspection area, for receiving the x-rays from the target material; means adjacent to the x-ray phosphor for receiving light photons produced by the x-ray phosphor as an image of a structure included in the inspection area and for producing an image of the structure.